In order to perform mass spectrometry, it is necessary to extract from a sample a target substance in the form of a gas-phase ion, using an ionization device. Various ionization methods have been developed, such as electrospray ionization (ESI) capable of ionizing polar substances such as proteins; atmospheric pressure chemical ionization (APCI) capable of ionizing low-polarity substances; atmospheric pressure photoionization (APPI) whereby substance-selective ionization can be performed by irradiation of light absorbed by a specific substance; and matrix assisted laser desorption/ionization (MALDI) whereby a solid sample in which a matrix and a sample are mixed is dropped onto a plate and irradiated with laser for ionization.
Among others, ESI is being widely used in combination with a liquid chromatography-mass spectrometry (LC-MS) device to ionize a wide range of substances. In ESI, some of sample microparticles formed by a spray that failed to be completely desolvated may be discarded without being fed into the mass spectrometry unit, or may enter the mass spectrometry unit in the form of the original microparticles and detected as noise. In order to increase the sensitivity of mass spectrometry, it is necessary to increase the amount of ion that enters the mass spectrometry unit and to reduce noise by improving the efficiency of desolvation of the sample microparticles produced by spraying and increasing ionization efficiency.
In order to improve solvent removal efficiency, a method has been devised which promotes the vaporization of solvent from the sample microparticles using various heating means. For example, sample droplets sprayed by ESI are irradiated with light including infrared light to evaporate solvent from the droplets (Patent Literature 1). In another example, a spray tip distal end is irradiated with laser for heating (Patent Literature 2). In this example, a spray tip made from a material that does not absorb laser, such as diamond, is used. In another method, a sample solution attached to a needle or the needle itself is irradiated with laser light for ionization (Patent Literature 3). In yet another method, a heating gas is introduced into the spray area to heat the sample microparticles (Patent Literature 4). Another example uses an optical fiber to irradiate a sample on a sample holder with laser light to ionize a target substance in the sample (Patent Literature 5).